Circuits in high performance radio frequency (RF) systems such as but not limited to wireless communications devices often require a regulated supply voltage. Voltage regulators are typically used to regulate the supply voltage. In some RF systems, more than one voltage regulator may be required. In other applications with spatial limitations, multiple circuits may share the same voltage regulator. For example, a voltage-controlled oscillator (VCO) circuit and a mixer circuit may share the same regulated supply. In this configuration, noise from the mixer circuit often appears at the output of the VCO circuit and vice-versa.
Referring now to FIG. 1, an exemplary voltage regulator 10 includes an operational amplifier (opamp) 12 and a PMOS transistor 14. An inverting input of the opamp 12 receives a reference voltage signal 16 and a non-inverting input of the opamp 12 receives a feedback signal 18. The opamp 12 generates an output voltage signal 20 that is based on a difference between the reference voltage signal 16 and the feedback signal 18.
The output voltage signal 20 is input to a gate of the PMOS transistor 14. A source of the PMOS transistor 14 is connected to a supply voltage 22. A drain of the PMOS transistor 14 is connected to the non-inverting input of the opamp 12 to provide the feedback signal 18. The voltage regulator 10 outputs a regulated signal 26 to an RF subcircuit 28 of a RF system 30. When supplying a single RF subcircuit 28, the regulated signal 26 is stable and constant. When a single voltage regulator supplies more than one RF subcircuit, noise or crosstalk from one of the RF sub-circuits may appear in the output of the other RF sub-circuit.
Referring now to FIG. 2, separate voltage regulators may be used for each subcircuit to eliminate the noise. The RF circuit 30 includes n RF subcircuits 28-1, . . . , 28-n that require voltage regulation. Voltage regulators 10-1, . . . , 10-n are provided for each RF subcircuit 28-1, . . . , 28-n, respectively. The voltage regulators 10-1, . . . , 10-n include opamps 12-1, . . . , 12-n, and transistors 14-1, . . . , 14n, respectively. Feedback signals 18-1, . . . , 18-n are generated as described above. When multiple voltage regulators 10-1, . . . , 10n are used, the spatial requirements and current dissipation of the RF system 30 increase.